In vitro cellular immune functions and lymphocyte subsets are being studied in a masked method in patients with ocular toxoplasmosis, pars planitis, Behcet's disease, geographic choroiditis, and chorioretinitis of unknown origin. Crude ocular antigens, as well as purified uveitogenic soluble antigen (S-antigen) and IRBP of the retina, are being used in a lymphocyte microculture technique to evaluate the presence of cellular immune memory to ocular tissues. In addition, purified antigens from the toxoplasmosis organism are also being tested in this in vitro system. A subgroup of patients with posterior uveitis has been identified as having this immunologic memory. Lymphocyte subsets in the blood and in the eye are being defined in these patients by monoclonal antibodies. These results shed light on the basic mechanisms of uveitis and may be used as a guide for specific immunologic therapy. In a small group of selected patients, chorioretinal biopsies are performed to evaluate the on-going ocular immune response.